A vehicle designed to provide switching operations for switching between a four-wheel drive mode and a two-wheel drive mode and switching a differential mechanism from a locked position to an unlocked position or vice versa is put in practical use. A power transmission device configured to perform these switching operations collectively is known as disclosed in JP-A-2008-279880.
As for a power transmission device disclosed in JP-A-2008-279880, a rotary member is rotated by a motor. The rotary member rotates a connection member via a spring. The connection member rotates an actuating shaft. The actuating shaft has a first cam groove formed thereon to move a first folk in an axial direction of the actuating shaft. The movement of the first folk moves a first sleeve to switch a positive clutch from a connected position to a disconnected position for switching a four-wheel drive mode to a two-wheel drive mode. The actuating shaft has a second cam groove formed at a distal end thereof to move a second folk in a direction perpendicular to the axis of the shaft. The movement of the second folk moves a second sleeve to insert a pin into a cam surface of a differential mechanism to thereby lock the differential mechanism. The rotational elements and the sliding elements of the power transmission device are placed in actuating oil for lubricating these elements to reduce resistance to rotational movement of the rotational elements or sliding movement of the sliding elements.
In the power transmission device of JP-A-2008-279880, the spring is interposed between the rotary member and the connection member. The spring is deformed for a period of time until the positive clutch is brought to the connected position or the pin is inserted into the cam surface. By having this period of time until the positive clutch is brought to the connected position or the pin is inserted into the cam surface, that is, it is possible to prevent the motor from having an excessive load. The mechanism for creating such a time period is called “lost motion mechanism”.
The actuating oil varies in viscosity depending upon temperature. The actuating oil hardens as the motor starts in a cold weather area. In this instance, the connection member tends not to rotate even when the rotary member rotates. For this reason, the motor runs for a long period of time, thereby compressing the spring greatly. As a result, a load on the motor increases.
From a standpoint of protection of the motor, it is desirable to shorten a running time of the motor and reduce a load on the motor even in a cold weather area.